Helloworld Program in java

Helloworld Program in java
 /* The HelloWorldApp class implements an application that simply displays "Hello World!" to the standard output. */
   public class HelloWorldApp {
 public static void main(String[] args) {
 //Display the string. No global main
 System.out.println(“Hello World”);
 }
 }

  To save your program, move to File menu and choose save as option.
 Save your program as “HelloWorldApp.java” in some directory. Make sure to add double quotes around class name while saving your program. For this example create a folder known as “examples” in D: drive
Note: Name of file must match the name of the public class in the file (at line 4). Moreover, it is case sensitive. For example, if your class name is MyClass, than file name must be MyClass. Otherwise the Java compiler will refuse to compile the program.
For the post, we assume that program is saved in D:\examples directory.

HelloWorldApp Described
Lines 1-3
Like in C++, You can add multiple line comments that are ignored by the compiler.
Lines 4
Line 4 declares the class name as HelloWorldApp. In java, every line of code must reside inside class. This is also the name of our program (HelloWorldApp.java). The compiler creates the HelloWorldApp.class if this program successfully gets compiled.
Lines 5
Line 5 is where the program execution starts. The java interpreter must find this defined exactly as given or it will refuse to run the program. (However you can change the name of parameter that is passed to main. i.e. you can write String[] argv or String[] some Param instead of String[] args)
Other programming languages, notably C++ also use the main( ) declaration as the starting point for execution. However the main function in C++ is global and reside outside of all classes where as in Java the main function must reside inside a class. In java there are no global variables or functions. The various parts of this main function declaration will be covered at the end of this handout.
Lines 6
Again like C++, you can also add single line comment
Lines 7
Line 7 illustrates the method call. The println( ) method is used to print something on the console. In this example println( ) method takes a string argument and writes it to the standard output i.e. console.
Lines 8-9
Line 8-9 of the program, the two braces, close the method main( ) and the class
HelloWorldApp respectively.


Compiling and Running HelloWorldApp
1.Open the command prompt from Start Æ Program Files Æ Accessories. OR
alternatively you can write cmd in the run command window.
2. Write cd.. to came out from any folder, and cd [folder name] to move inside the specified directory. To move from one drive to another, use [Drive Letter]: See figure given below
3. After reaching to the folder or directory that contains your source code, in our case
HelloWorldApp.java.
4. Use “javac” on the command line to compile the source file (“.java” file).
D:\examples> javac HelloWorld.java
5. If program gets successfully compiled, it will create a new file in the same directory named HelloWorldApp.class that contains the byte-code.
6. Use “java” on the command line to run the compiled .class file. Note “.class” would be added with the file name.
D:\examples> java HelloWorld
7. You can see the Hello World would be printed on the console. Hurrah! You are successful in writing, compiling and executing your first program in java ☺
Points to Remember
 Recompile the class after making any changes
 Save your program before compilation
 Only run that class using java command that contains the main method, because program executions
always starts
An Idiom Explained

You will see the following line of code often:
– public static void main(String args[]) { …}
• About main()
“main” is the function from which your program starts
Why public?
Since main method is called by the JVM that is why it is kept public so that it is accessible from outside. Remember private methods are only accessible from within the class
Why static?
Every Java program starts when the JRE (Java Run Time Environment) calls the main method of that program. If main is not static then the JRE have to create an object of the class in which main method is present and call the main method on that object (In OOP based languages method are called using the name of object if they are not static). It is made static so that the JRE can call it without creating an object.
Also to ensure that there is only one copy of the main method per class
Why void?
• Indicates that main ( ) does not return anything.
What is String args[] ?
Way of specifying input (often called command-line arguments) at startup of application.